Sport Stacking involves stacking specially designed cups in pre-determined sequences as fast as possible and is often incorporated into school physical education curriculums because it inherently increases student's hand eye coordination and reaction time. In addition to specialized cups, a specialized stacking surface is also usually employed that provides friction to help prevent sliding of the cups. The specialized surface is typically comprised of a neoprene foundation with a polyester fabric bonded to the top surface of the foundation, a composition often employed in the construction of computer mouse pads. Specialized cups are required to practice the sport and the cups must have certain features that allow them to slide easily against one another and against the stacking surface.
More specifically, sport stacking cups exhibit certain properties that are desirable for fast stacking such as smooth top and bottom surfaces wherein the cups can be dragged across the stacking surface without the top or bottom surface of the cup catching on the stacking surface. If a cup catches, drags or is in any way impeded by the stacking surface, the cup stacker will need to apply additional force to pull the cup into position which may slow the stacking process. Further, as one skilled in the art will appreciate, a rough surface may cause a stacked or placed cup to tip.
Cups used for sport stacking are typically injection molded from polymers, i.e. plastics. Injection molding is a popular manufacturing technique for making parts from thermoplastic material. Molten plastic is injected at high pressure into a mold that is inverse of the desired shape. The mold is made of metal, either steel or aluminum, and precision machine to form the features of the desired part. Injection molding is widely used for manufacturing a variety of parts, from the smallest component to the entire body panels of automobiles. It is the most common method of production with some commonly made items including bottle caps and outdoor furniture.
The molds used in injection molding generally include slidingly mated moving elements. During the use, wear between the moving elements create a gap therebetween that is apt to receive molten plastic during the molding process. After the molded part is removed the excess plastic forms unwanted protrusions on the part, typically known as “flash” to those skilled in the art. The occurrence of flash may be reduced by maintaining the mold periodically which involves replacing or refinishing the surfaces of certain parts of the mold but some degree of flash is usually present, even with a properly maintained mold.
In order to provide a stacking cup with smooth surface, the flash must be removed in a post processing application. Since such an operation is often time consuming, labor intensive, and costly, it is desirable to mold parts in such a way that post processing of parts is minimized or eliminated. For example, trimming to remove unwanted flash is not desirable since the labor involved in doing so may make the parts uneconomical to produce and also because under or over trimming a part may result in a reject. Post-molding inspection associated with flash further reduces the cost effectiveness of the manufacturing process.
Molds used for forming a cup are generally comprised of a core, which can be any profile, but often have a conical outer surface that forms the inner surface of the finished cup. The outer surface of the cup is defined by a cavity that is positioned a predetermined distance from the core to define the thickness of the finished cup. During forming, heated polymer is injected into the mold thereby filling the space provided between the core and the cavity. Subsequent to injection, the part is allowed to cool, i.e. cure, so that it can be removed from the mold. During this cooling process, the cup will necessarily contract about the core. Removal of the cup begins with removal of the cavity thereby exposing the outer surface of the cup. A stripper plate, which was previously positioned beneath the cavity, is transitioned relative to the stationary core thereby disengaging the cup from the mold. This movement is a source of the wear and tear on both a stripper plate and the core that potentially causes a gap. The gap, as described above, is very apt to receive material that escapes from the mold thereby creating flash.
As mentioned above, the construction of an injection mold for making a cup is typically such that a moving plate or “stripper” is required for removing the cup from the mold after the polymer is injected therein and cured. Stacking cups often include a thickened lip about their open end, similar to common drinking cups, that strengthens the opened end of the cup (“bottom” as used herein) and provides a detente that engages a lip of an adjacent cup when a plurality of cups are stacked within each other. Given the lip configuration of sport stacking cups, the joint between the stripper and the inner portion of the mold (also known as the “core”) is located directly on the bottom of the lip of the cup. To disengage the cup from the core, the stripper plate moves relative to the core, therefore, the joint between the core and the stripper plate is non-static and subject to the wear that is often attributed to flash being present on the bottom surface of the cup. Flash is detrimental to the cups for use in sport stacking because the flash will often catch on the stacking surface. Therefore, a method for eliminating the detrimental effects of flash at this point on the cups is of great advantage in the design of cups for sport stacking. Indeed such a method has been discovered.
Thus it is a long felt need in the field of injection molding of cups to provide a method of reducing flash, specifically flash adjacent to the bottom edge of the cup. The following disclosure describes and improved method of injection molding a cup to be used in cup stack competitions that decreases the amount of flash associated with the bottom lip thereof that would come in contact with the stacking surface.